The present invention relates to a security element in the form of a thread or strip to be embedded in a security document comprising a carrier sheet and at least one marking substance applied thereto. e.g. in the form of a coating, to allow for visual and/or machine detection of authenticity, and to a method for producing and testing the same.
It is a known measure to protect security documents by embedding in these documents security elements which either have special machine detectable physical properties or are designed in terms of their visual appearance so as to serve as an authenticity feature for the security document. Such security elements are e.g. threads or strips directly embedded in the paper ply as it is being formed during the manufacture of the paper.
Such a security thread is known e.g. from German laid open print no. 27 54 267, whereby a carrier sheet made of polyester or cellulose is coated with various materials. The visually and/or machine readable authenticity features are e.g. an aluminized coating, a magnetic coating, a coating of X-ray absorbent material and/or luminescent material.
This publication shows various structures of such security threads which differ in their layer arrangement and/or the materials used thereby. However, all examples shown therein have at least one, usually even several, of the disadvantages stated below.
For example, the layer materials susceptible to mechanical abrasion and atmospheric influences, such as the metal coating, usually form the outside of the thread. However, mechanical stresses during production of the thread can easily destroy the metal coating, and during subsequent use it is often the case that the metal coaxing loses its original shine and eventually turns dull. In serious cases the metal layer may even be completely interrupted, so that the security thread loses not only its reflectiveness but also its machine testable electrical conductivity over the entire length of the thread.
Furthermore, the structure is usually asymmetrical and this asymmetry leads to curling of the thread and the formation of garlands. Thus, the thread cannot always be embedded readily in the same flat position in the fibrous layer of a paper ply being formed. The areas of the paper web in which the security thread does not come to lie on the plane of the paper ply must subsequently be eliminated as being useless. This reject rate leads to higher costs for the paper production.
Finally, manufacturing errors in the coating process cannot be ruled out. For example, there may be wide fluctuations in the layer thickness and also complete interruptions In the coating. Such interruptions in a metal layer, for example, result in an interruption of the electrical conductivity. These security documents must also be eliminated as rejects since these irregularities prevent reliable detection of authenticity.